Date: 9/5/2024

White Bear Photonics, LLC

2157 Randy Ave

White Bear Lake, MN 55110

info@whitebearphotonics.com

Summary

FTIR “not scanning” or “alignment failed” is a common failure and in most cases is due to a dead laser, provided the optics and electronics are fully functional. The second most common root cause for this problem is the KBR optics are “fogged” due to humidity exposure. Potassium Bromide (KBR) is used extensively for FTIR optics due to its cost, and wide spectral range. The downside is its susceptibility to humidity since it is water soluble. If the FTIR was stored (neglected) and/or hydrated (high humidity), here are three components that will need to be replaced: Beam splitter/Interferometer, Sample Port Windows, and the DTGS room temperature IR detector.

During the auto align process in the Diagnostics tab, there are two critical steps that need to be completed before the system is operational: tuning the laser, and optimizing the amplitude of the IR beam.

In first step of the auto alignment, the laser detector (just outside the Interferometer) has three detectors and three corresponding amplifiers, (X, Y and R signals). As the mirror moves there are three sine waves generated by the laser interference fringes sweep across each detector. Each amplifier gain is automatically adjusted for the optimum sine wave phase and amplitude. If the optimum cannot be achieved (due to a bad laser or fogged optics), the auto align will fail. If the sine waves are detectable, you will see a red line with a small interferogram peak appear during this process. If there is no red line present at any time during the auto alignment, this means the laser signal is simply undetectable (for various reasons).

The second half of the automatic alignment process is the optimizing the interferogram amplitude which done by the galvo mirror in the interferometer that points the IR beam to the optimal position on the IR detector. During this process you will notice the interferogram maximum is changing and once the maximum is determined, and the galvo voltages are set. When this step is finished, the alignment has been successfully completed, and the FTIR is capable of scanning. If there is a red line without a peak when completed, then the IR source is not ON or is dead. In other words, the interferometer will be scanning with no IR beam present, and therefore only be a flat line with no peak present.

Assessment

1. Laser state. This is to verify the laser is on (red light). The best way to prove this is to remove the cover and inspect the beam exiting the laser head. If there is no red light at all, then the laser must be replaced.
2. The manufacturing date of the laser. Typical lifetime of a HeNe laser is 5-7 years. This is not a hard number since some laser will last 10 years others will last 1 year (or less). HeNe lasers will either get weak over time (low power) or fail outright (no light). Weak lasers are problematic where the FTIR will have intermittent bad scans and more notably high laser Gain values for X, Y, and R. If the FTIR is scanning with intermittent bad scans, you can check the laser gains by closing Ominic and going into Cdrive->Programs->Ominic-> then open the NicDiag or ADiag application then -> Laser->Laser Gains. The gains for X, Y, and R should all be less than 100 for normal operation. If the any of the gains are 240 the system will be unstable.
3. KBR Beam Splitter. New KBR windows and beam splitters will appear perfectly clear (same as glass optics). KBR exposed to high humidity will appear to have a “fogged” or crazed surface. When the HeNe laser is ON, the red beam will glow and scatter light on damaged KBR surfaces (e.g., like frosted glass). Even though you can still have red light hit the laser detector, the phase of the light will be scrambled and impossible to generate the necessary sine waves for X, Y, and R. If the beamsplitter in the interferometer is damaged from humidity, it needs to be replaced. KBR optics cannot be cleaned, they are very soft and easily damaged by anything touching the surface. For the research grade FTIRS such as the Magna, Nexus, 6700, and IS50 models, the beam splitter is removable from the interferometer. For these models it is a matter of replacing just the beam splitter (not the entire interferometer). For the other Thermo Nicolet FTIRs: Avatar series, IS5, IS10, IS20, IN10, and Summit models, the entire interferometer module will need to be replaced if the KBR beam splitter is damaged. For these models, the KBR beam splitter is not removable (from the interferometer) or interchangeable.
4. Sample Port Windows. Sample port windows are located just to the right and left inside the sample compartment (where the IR beam enters/exits) and are necessary to keep the FTIR sealed from the outside air (with high humidity). These windows are usually coated with Pyraline which is a moisture resistant coating so outside air will not fog the KBR surface. The Pyraline coating however is limited to about 70% RH, so damage still can occur to these windows with high humidity exposure. These windows can easily be inspected by shining a flashlight on the surface and inspect for any fogging or crazing.
5. Room Temperature DTGS Detector. The room temperature IR detector will also have a KBR window. This can be inspected with a flashlight by examining the integrity of window surface (fogged or crazed). Any fogging will cause scattering and a reduction in amplitude of the signal and will need to be replaced. This visual inspection is not reliable upon full testing (e.g., Valpro Certification). Hydrated detectors can easily fail noise specs even though the surface of the window shows no damage.